The dream Castlevania game... I have no idea, but for me the ideal Castlevania game must have clear and crisp graphic style. Dark and horror style also OK for me. (CV The Lecarde Chronicles and Rosenkreuzstilette/RKS Freudenstachel are very good examples since I don't care about them being fan-made games!)
The stage setting, hardest difficulty must have legit control by producer and developer stuff in order to get the game not to be too frustrating (I can't stand the difficulty of CV Adventure Rebirth Hard Mode stage 5 and Hard final boss because of the lack of reasonable stage/enemy setting and recover rate. I tried my best to get through,with my 350+ Christopher buddies fallen just in final boss fight, FINALLY vanquished the horror and it's still a pain in the @$$ even in MUCH lower frame-rate thanks to emulator. This kind of frustrating should be ILLEGAL. It's really important to eliminate similar problems in a possible 2D side scrolling action game as much as possible)
Also the strange hit-detection of player's move and attack of enemies must be eliminated as much as possible. (Again, RKSF did an awesome job there. I don't even believe it and forgot it being an independent or fan-made game.) Or just drag the exact engine used by RKS/RKSF and changed to a Castlevania skin with Classic CV gameplay. If one did so the result would be awesome! (The reason why RKS is stopping for now when it's REALLY close to form a great 2D side scrolling action trilogy IS the planner WOMI didn't do anything except drawing some unrelated porno pictures for a completely separate personal project. Not doing. NOT doing. WOMI, you are damned.)
What kind of mechanics would you want incorporated...
I think Richter's Rondo style would be very cool, plus some useful and convenient arrangement and martial art moves like sliding, multi-directional whip skill. The style similar to CV Chronicles PS1 version with arrangement also OK for me (In fact CV Chronicles PS1 version was one of the best video game I've ever played, or THE game have the most possibility to approach my "dream" Castlevania game, I've been keeping 40+loops just for Original mode of Japanese version; The ONLY problem is that the format of the game is old-style and barely any of my friends satisfying that)
A separate hidden character which has marriage of abilities from Maxim Kischine and Mega Man Zero would be awesome. Powerful, but not to be too overpowered like Maxim
Who would you want to develop it?...
I think probably the stuff from CV X68000 team, DS CV team, Mega Man ZX team or RKS stuff are OK for me.
For publish, I don't care about the publish company being big or small or just without such a thing like that. BRING ONE GOOD PRODUCT AND IT'S ENOUGH FOR ME TO BE SATISFIED.
2D, 2.5D, 3D, VR?...
No doubt the 2D sprite-based style. The unlimited possibility just setting there
3D Castlevania may be cool but there are always some limits here and there, though CV64 format with modern arrangement would be good
HD remake or brand new title? Main canon, side canon?...
A brand new title with side canon that won't set against the existing canon would be very interesting. I like to see the plot like the "Dracula" which the Belmont meet at first is not real Dracula but just an impostor hired by him, though his dressing, wine glass, his throne room with tower bridge, and anything are just like what the real Dracula once did (aka CV64), the first castle would be simple and straightforward and once the impostor defeated there is another bigger castle to explore.
Other things...
For story dialogue/cut scenes, the quality equal to Mega Man Maverick Hunter X (English version! I love the English VA and English version OP song, for the English OP I still won't get bored after looping 700+times in during several years. REALLY hope there is another game which has THE quality of opening song) is the best
No too many update patches or annoying revisions. A good game should be polished enough when it's first-out, just like those form GBA/DS era
For the control of quality I think the producer and planner stuff should become the master player of the game first, give players an "I can also do it" message and engage the active power of most players
An Art/Boss Contest could be hold, but for such a "dream" game the commercial activities must avoid being too topic by mainstream or net whispers (Or just get rid of commercial activities just like what lots fan-game did). Under-light means high stress, thus being the source of high risk.